HC-SCRs are sometimes also referred to in the literature as non-selective catalytic reduction (NSCR) catalysts, lean NOx catalysts (LNC), lean NOx reduction catalysts, “DeNOx catalysts.” and NOx occluding catalysts.
In hydrocarbon selective catalytic reduction, hydrocarbons (HC) react with nitrogen oxides (NOx), rather than oxygen (O2), to form nitrogen (N2), carbon dioxide (CO2) and water (H2O) according to Reaction (1):{HC}+NOx→N2+CO2+H2O  (1)
The competitive, non-selective reaction with oxygen is given by Reaction (2):{HC}+NOx→N2+CO2+H2O  (2)
The most effective HC-SCR catalysts used to selectively promote the desired reaction (1) are Pt/Al2O3, Cu exchanged ZSM-5 and Ag/Al2O3. Ag/Al2O3 catalysts operate at higher temperatures and over a broad temperature range, and have recently shown promise in vehicle testing (Klingstedt et al., Topics in Catalysis, 30/31, 2004, 27 and Lindfors et al., Topics in Catalysis, 28, 2004, 185, the entire contents of which are incorporated herein by reference).
All of these catalysts exhibit high activity for the selective reduction of NOx by hydrocarbons, including long chain alkane and diesel fuel, but each type of catalyst suffers from some form of limitation in use. Pt/Al2O3 catalysts display lower NOx conversion and lower selectivity towards nitrogen; N2O (conversion >60%) is a basic product. Additionally, the HC-SCR activity window of Pt/Al2O3 catalysts is limited to low temperatures (about 150-250° C.). Generally speaking, Cu/ZSM-5 catalysts can suffer from thermal deactivation due to copper sintering and dealumination of the zeolite. Ag/Al2O3 catalysts are tolerant to hydrothermal aging, but can suffer from chemical deactivation caused by coking or sulphation. We understand that the relatively poor performance of Pt/Al2O3 catalysts and the relatively poor activity of Cu/ZSM-5 and Ag/Al2O3 HC-SCR catalysts once aged, has so far been insufficient to allow for their widespread implementation (Konig et al., Topics in Catalysis, 28, 2004, 99, incorporated herein by reference).
Coking is not a significant factor in the activity of any HC-SCR catalyst at higher temperatures since above approximately 400° C. any carbon present will be burnt to form CO2 thereby leaving the catalyst surface available for reactions to take place thereon. As such it is important to differentiate between the absolute activity of any particular HC-SCR catalyst, and the reduction in activity that coking may result in. An increase in the absolute activity of any particular HC-SCR catalyst will not necessarily be the result of a concomitant reduction in coking.
Nonetheless, coking does have a significant effect upon the HC-SCR activity of Ag/Al2O3 catalysts at lower temperatures and therefore a means was devised to limit access of the hydrocarbon species responsible for coke deposition to the catalyst to minimise coking deactivation. Said means involved modifying the Ag/Al2O3 catalyst formulation, and is disclosed in WO 2005/016496 (incorporated herein by reference). Specifically, it was disclosed that by combining known HC-SCR catalysts with a partial oxidation catalyst (POC), it was possible to suppress or avoid low temperature coke formation. In the WO 2005/016496 invention, the POC helps to prevent coking by promoting the partial oxidation of hydrocarbons in the exhaust gas of a lean-burn internal combustion engine to carbon monoxide (CO), hydrogen gas (H2) and partially oxygenated hydrocarbon species. Therefore the heavy hydrocarbon species present in the exhaust gas are partially oxidised to smaller, more reactive species prior to contacting the downstream HC-SCR catalyst.